This invention relates to an improved silica gel sorption composition for use in this layer chromatography.
The increasing utilization of thin layer chromatography (TLC) has mandated the development of better separating materials, with greater value placed upon better efficiency with regard to effective separating ability and lowering the limits of detection as well as with regard to the reproducibility of results.
In thin layer chromatography there are usually employed carrier materials coated with sorption agents; silica gel layers on glass plates or on foils have found the greatest practical use. The particle sizes of these sorption agents, in the case of the best commercially available preparations, are on the order of about 5 to 25 .mu.m in diameter wherein the average particle sizes vary approximately between 10 and 15 .mu.m. Smaller particle sizes have generally been regarded as disadvantageous since with decreasing particle size the flow rate of the elution agent decreases considerably so that the necessary separation times become too long; simultaneously, diffusion increases and separation ability is reduced. Therefore, even separation materials with coarse particles which are said to give a separation ability almost equal in value to the otherwise conventional TLC plates have been suggested, e.g., in Deutsche Apothekerzeitung, 113, page 791, 1973.
Chromatographic separations on ultra-thin silica gel and aluminum oxide layers have also been suggested, e.g., see Z. Chem. 1972, pages 152 and 153. As chromatographic material, there is used the undefined very fine grain portion which happens to remain adhering to the plate after rinsing of the carrier material. Therefore, reproducible and comparable results are not achievable with such layers. Furthermore, such ultra-thin layers are unsuitable for mechanical evaluation with the help of the most widely used remission measurements. The limitation to solely transmission measurements is a considerable disadvantage, as is the reduced suction force which permits only a horizontal development.
In order to avoid some of the above-indicated difficulties so-called thin film chromatography has been suggested, e.g., as described in German Pat. No. 1,943,304. Thin layers (&lt;10 .mu.m) applied by vapor deposition, for example indium oxide, are employed as the stationary phase. However, such vapor deposited layers exhibit an extremely small capillary action, from which result very flow rate coefficients, very slow running times and very low running heights of only a few mm. Therefore, only a very limited number of substances can be satisfactorily separated by this technique.
Thus, there still exists a need for efficient thin layer chromatography separating materials with which good separations can be carried out quickly and with good sensitivity. The present invention fills such a need.